The first photos of Mohammed Ali Malek, the alleged captain of the fishing vessel that sank off the Libyan coast on Sunday, drowning about 800 migrants, showed him biting his nails, nervously, as he was approached by Italian police.

It was just before midnight on Monday. Mr. Malek, a bearded 27-year-old Tunisian, was on the Italian Coast Guard ship, the Gregoretti, and was about to get arrested. Shortly thereafter, when he was on Sicilian soil at the port of Catania, he was charged with reckless multiple manslaughter, causing a shipwreck and aiding illegal immigration.

Also arrested was Mr. Malek's alleged crewman, Mahmud Bikhit, 25, a Syrian. They were identified by the other survivors as the traffickers who took them on the deadly voyage that started in Libya in a decrepit 20-metre fishing boat and sank overnight on Sunday, well before it reached Italy, the skipper's intended destination.

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The final death toll from the biggest mass drowning in the Mediterranean since the Second World War is still not known, though Italian prosecutors say it could be 900 or higher.

In a statement Tuesday, the prosecutors said the captain mistakenly smashed the boat into the Portuguese merchant ship that had come to the migrants' rescue. The migrants' boat, dangerously overloaded, was already off balance and the collision evidently triggered a stampede to one side, tipping the boat and probably downing everyone confined in the lower and middle decks of the three-deck, wooden vessel.

The prosecutors said "many migrants, including women and children, were locked in the hold." The survivors told horrific stories about the shipwreck. According to Italy's La Repubblica newspaper, some of the survivors clung to floating bodies to avoid drowning and screamed to raise the attention of rescuers from the Italian and Maltese Coast Guards.

Seamen who participated in the rescue efforts told tales of near-miraculous rescues.

"We found, literally, a floating cemetery. Bodies were everywhere. With the dinghies we had to literally slalom among the corpses," said Enrico Vitello, a 22-year-old medic from the Order of Malta.

Hearing screams, they killed the engines and shined a spotlight, locating a migrant floating in the sea.

"We got close by and rescued him," said Giuseppe Pomilla, a 30-year-old medic. "He asked our names and where we were from. We told him we were Italians and came to rescue him. He was so happy."

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Soon after, a boy floating in the sea grabbed their attention.

"We couldn't understand if he was alive or dead. He had his eyes wide open looking at us. He was not blinking, not moving or talking. We only realized he was alive when he grabbed us suddenly," Mr. Pomilla said. When they took him on board, he "exploded in tears," the medic said.

There were 28 survivors; only 24 bodies have been recovered. According to the UNHCR, the United Nations' refugee agency, 350 passengers on the doomed boat were Eritreans. Many of the rest were from Syria, Somalia, Mali, Senegal, Gambia, Ivory Coast and Ethiopia.

"From the information and available accounts we've had, the UNHCR now believes the number of fatalities to have been over 800, making this the deadliest incident in the Mediterranean we have ever recorded," the agency said Tuesday.

The UNHCR staff said the survivors coming off the Italian Coast Guard ship were tired and nervous. "The survivors looked exhausted, fragile, astonished to see so many people waiting for them," UNHCR spokeswoman Carlotta Sami said from Catania. "They will need psychological support."

In Geneva, the International Organization for Migration calculated that, so far in 2015, more than 1,750 migrants have died in the Mediterranean. That's more than 30 times the number recorded over the same period last year, when 56 migrant deaths were confirmed. With distress calls and new shipwrecks reported almost every day, the human toll could rise substantially in the coming weeks as the weather improves and the Libyan traffickers send fleets of rickety migrants' boats to sea.

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On Monday, the Italian Coast Guard said it had rescued 638 migrants from dinghies in six separate operations. On the same day, three migrants died when their vessel smashed into shoals off the Greek island of Rhodes, in plain view of resort patrons.

On Tuesday, European leaders reiterated the promise to devote more resources to the European Union's Triton patrol program, the maritime sweep that replaced Italy's far larger Mare Nostrum patrols late last year. The migrant tragedies and how to stop them will dominate an EU leaders' summit in Brussels on Thursday.

"In terms of funding need for rescue missions at sea, I expect that we don't ask questions on Thursday but offer funding," Austrian Chancellor Werner Faymann said at a news conference Tuesday.

But the plan to crack down on Libya's human traffickers and smugglers is a work in progress. Italian Prime Minister Matteo Renzi said that his government was studying the possibility of "targeted interventions" against the smugglers. European governments, however, will be wary of direct military intervention because the traffickers are said to be well-established and well-armed, especially in the smuggling centres west of Tripoli.

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